This invention relates generally to a cardiac pacer circuit and more specifically to an improved demand-inhibit cardiac pacer circuit for artificially stimulating the heart muscle only in the absence or interruption of normal heart activity.
Demand-inhibit pacers are known in the art. Typically, demand-inhibit pacers detect the electrical signal emitted from the heart characterizing the depolarization of the ventricular muscle tissue, commonly termed the R-wave signal. Upon an indication of an R-wave signal being delivered by the heart muscle, the inhibit function of the pacer becomes operative to inhibit delivery of an artificial stimulating pulse to the heart muscle as long as natural heartbeats are occurring at a prescribed rate. In other words, the inhibit function is operative for a single cycle only of the heart activity. Once having sensed the occurrence of an R-wave signal, the inhibit function prevents or precludes delivery of an artificial pulse until sufficient time has elapsed so that the next succeeding R-wave signal is due. One such system is disclosed in the Berkovits U.S. Pat. No. 3,345,990.
Demand-inhibit pacers have traditionally suffered from one serious deficiency. While the normal function of these devices is to inhibit the generation of an artificial stimulating pulse during periods of normal heart activity, the prior art demand-inhibit pacers have sometimes been unable to distinguish between the signal from a natural heartbeat and stray electrical noise which may be picked up by the implanted electrodes from external sources. In other words, prior art pacer devices, when subjected to external noise at certain frequency bands, may produce an inhibit signal to disable the generation and delivery of the artificial impulse. Should this happen at a time that normal heart activity has ceased, it could prove fatal to the patient.
This problem has been recognized, and approaches have been advanced for attempting to solve it. More specifically, in the Berkovits U.S. Pat. No. 3,528,428, there is disclosed a circuit which is capable of operating in two modes. In the absence of external noise, the circuit described in U.S. Pat. No. 3,528,428 operates to produce an inhibit signal for disabling the application of artificial stimulating pulses to the heart when normal heart activity is sensed. However, when external noise is detected by the system, the circuit shifts to a second and asynchronous mode of operation wherein artificial stimulating pulses are generated at a fixed, predetermined rate irrespective of normal heart activity. While the circuit just described provides a solution to the problem of insuring that external noise will not continuously inhibit the generation of heart stimulating signals, it does so at the expense of producing such heart stimulating signals even when they may not be required because of continuing normal heart activity.